Drying of wood

ABSTRACT

Wood in the piece, initially containing an undesirable amount of water (moisture) may rapidly yet safely be dried as follows: The wood is heated, in a closed container, under reduced pressure at a temperature not exceeding 90*C., in a bath of a waterimmiscible drying medium having a boiling point above 150*C. but not above 250*C., for a period of some hours. In the operation, the extent of the partial pressure is adjusted to minimize distillation of drying medium whilst promoting evaporation of water from the wood. To minimize &#39;&#39;&#39;&#39;loss&#39;&#39;&#39;&#39; of drying medium left in and on the treated wood, the wood may, after removal of the bath of medium from the container, be held for some time, under vacuum, in the container. Steam-medium so driven off is conducted to a separate space where medium is separated from water and returned to the drying process.

United States Patent [191 Hager [111 3,811,200 May 21, 1974 DRYING OF WOOD [75] Inventor: Bror Olof Hager, Djursholm,

Sweden [73] Assignee: Hager Aktiebolag, Stockholm,

Sweden [22] Filed: Feb. 22, 1973 [21] Appl. No.: 334,938

[30] Foreign Application Priority Data 3,685,959 8/1972 Dunn, Jr. et al 34/95 FOREIGN PATENTS OR APPLICATIONS 620,883 5/196] Canada 34/95 Primary Examiner-John .l. Camby 5 7] ABSTRACT Wood in the piece, initially containing an undesirable amount of water (moisture) may rapidly yet safely be dried as follows: The wood is heated, in a closed container, under reduced pressure at a temperature not exceeding 90C., in a bath of a water-immiscible drying medium having a boiling point above 150C. but not above 250C, for a period of some hours. In the operation, the extent of the partial pressure is adjusted to minimize distillation of drying medium whilst promoting evaporation of water from the wood.

To minimize loss of drying medium left in and on the treated wood, the wood may, after removal of the bath of medium from the container, be held for some time, under vacuum, in the container.

Steam-medium so driven off is conducted to a separate space where medium is separated from water and returned to the drying process.

9 Claims, No Drawings DRYING F WOOD The present invention relates to an improvedprocess for a very fast yet mild drying of wood.

A fast drying of wood has two seemingly conflicting criteria: (a) there is needed a good supply of heat for the evaporation of the water, but above all, (b) it is essential that the heating of the wood in different respects be effected so mildly that no damage occurs to the wood. These criteria have proved to be difficult to meet.

When drying with air in conventional apparatus, it is necessary to be very careful in order not damage the wood. It is of special importance that the surface of the wood not dry too fast in relation to the inner parts of the wood; in such a case warpings, shakes and discolorations appear. Therefore, the temperature of the drying air medium must be kept low and, further, it ought to have some humidity. This results in a slow drying rate.

Even if these problems attendant upon air drying could become better controlled which does not seem possible the problems related to heat transfer remain. The heat transfer needed for a fast drying can probably not be effected if the drying medium is air. For this pur pose needed temperature differences between the drying medium and the wood will be definitely too great.

Technical advantages can be realized by drying with some other gases than air. These gases discharge a part of the atmospheric air pressure for the water-steam leaving the wood. It forms so to speak a vacuum whereby the boiling point of the water to be evaporated is decreased whereby the water in the form of steam leaves the wood faster. As a rule those gases have been used which, by condensation, have been able to be recovered after the drying operation.

In such case the drying fan principally be fulfilled faster and at a lower temperature than when air is used. The amount of needed heat in order to evaporate the water in the wood is of course not reduced, the heat of vaporization in both cases being the same. Even here there is a risk of a too fast drying of the surface of the wood with consequent damages thereto. Further, the gas just as in the case of air has a low heating capacity. It is therefore not an effective heat carrier. Drying with gas is more complicated than with air, and, as a whole, does not offer more evident advantages.

It is possible by some electrical methods to heat the inner parts of the wood directly. According to this expedient, it is possible to obtain a fast and rather even drying without damage to the product. These methods are, however, difficult to apply to larger pieces of timber; further, heretofore, the proposed methods are very expensive in use.

Drying methods according to which the wood is heated in different liquid media have been proposed, but they have not gained further use for different reasons, above all because of their indescribable influence on the quality of the wood so dried.

In connection with pressure preservation, drying and coloring of wood according to the Hager-process Swedish Patent 301,870 it has in later years been proved that drying wood with a high boiling oil is able to effect a surprisingly fast and harmless operation. By this process the wood absorbs some drying oil, but through special measures the absorbed part can be kept as small as possible. Retention of the oil can be brought down to an amount which is not more than desirable in order to obtain some desired effects of the Hagerprocess. However, in ordinary drying it may not be desirable to introduce even these small oil quantities in the wood.

The wood drying method according to the present invention has some resemblance to the Hager-process. Briefly described it is carried out in the following way.

The moist wood to be dried is introduced into a metal cylinder adapted to be closed. Into the cylinder there is also introduced a petroleum product (drying medium) with which the drying is carried out. It boils at about 200 C. Heat is supplied so that the petroleum product is kept at 80 C. Further, a high vacuum is applied so that the water in the wood boils away. The water-steam that leaves the wood and the accompanying evaporated drying medium are led to a coolercondenser. The condensate so obtained is then led to a collecting vessel. From this latter the drying medium is separated and taken care of for re-use.

When the wood has been dried to a wanted degree the drying medium is carried away from the cylinder. This is done under a maintained vacuum (reduced pressure). The vacuum in the cylinder is maintained even some time after the moment the drying medium has been caried away from it. Drying medium remaining in the wood surface is evaporated from the hot wood during this period. The wood is then ready for further treatment or use.

The different parts of this simplified description will now be discussed in further detail.

A suitable apparatus comprises an iron cylinder with an end cover which is possible to open and close. On the cylinder is a heating mantle so placed that the cylinder and its contents can be heated. The heating can also be effected for example via heating tubes within the cylinder, etc. As heat transfer there can for instance be used water, other liquid such as oil, or steam. The cylinder ought to be thermally insulated.

A conduit communicates between the cylinder and a container for drying medium, through which conduit the medium can be transported to or from the cylinder. It must be possible to carry away the drying medium when the cylinder is under vacuum, for example, via a suitable liquid pump or by gravity flow to an underlying evacuated container.

From the cylinder there leads, further, a tube for exiting vapors to a cooler wherein the eapors are condensed to water and liquid drying medium respectively.

From the cooler the condensates are led to a container where water and drying medium are separated from each other. The drying medium is taken care of. It is of small importance if a part of the water accompanies the medium on the contrary, separated water ought not to contain drying medium.

The necessary vacuum pump which evacuates the drying cylinder, the cooler and the separating container is suitably connected with the cooler or the container. Further, suitable measuring and controlling devices shall be present. Details in this apparatus can be such an amount that it is when in foaming condition (i.e., when during the drying period it is mixed with evaporating steam bubbles) covers the wood but no more. The drying medium should thus not completely fill the cylinder but the medium forms a free foaming surface over the wood from which surface water and drying medium leave in the form of vapors into the conduit to the cooler-condenser. Successive fillings of drying medium during the drying period are as a rule necessary.

As drying media there may be used liquids which are not miscible with water and which boil within a certain interval (have a suitable steam pressure curve). The boiling point down is limited by the fact that it shall be possible to heat the wood under vacuum without causing the drying medium to boil or foam too vigorously, resulting in a fast evaporation. Undue vigor would cause an unnecessary charge to the process; drying would be delayed; permanent fillings of the drying medium would be necessary; the consumption of heat would increase; and there would be larger amount of drying medium to recover.

It is possible to effect the drying with the use of a rather low boiling medium. Media boiling at l5 0l60 C. and even lower can be used. The drying media will be more practically useful first when the boiling point amounts to l80 C. The drying process per se is effected easier if the media have a high boiling point. The boiling point must, however, not be too high so that the media remain in the wood after the drying. On the contrary, they must be able to evaporate from the wood. The upper limit for the boiling point seems to be at or just under 250 C. From different view-points media boiling at between 200 and 250 C., preferably between 200 and 220 C., i.e. around 200 C. or a little over this, -are preferable.

Thedrying medium may for example be composed of petroleum products. Even different organic solvents can be used,- for example, such as contain chlorine. As a rule, the cheaper petroleum products are to be preferred for reason of economy. Further, such products ought to be selected which are odorless or in any case free from disagreeable smell. This is specially important if risks exist that small quantities of the high boiling parts of the media may be retained in the wood. The flash point of themedium should be as high as possible.

Among organic solvents the chlorinated products have the advantage of being rather resistant to fire. They are definitely more expensive than the simple petroleum products. Therefore, it is also rather important to recover them as completely as possible. A little lower boiling point of the substances can be chosen in order to facilitate their recovery. Even if the drying is carried out in a closed container the health hazard possibly caused by chlorinated products should be considered. When petroleum products are used, it may be advantageous to include chlorinated substances as additives in order to increase the flash point of the resulting medium.

Petroleum products having high aromatic contents, and other more specific solvents, will as a rule leach out incrustations (resins) from the wood. The media will, in these cases, often be strongly yellow colored. After the treatment of some timber loadings an equilibrium (saturation) is as a rule formed so that the medium leaves on the wood as much wood incrustation as is leached out from the wood. In spite of the fact that this leaching may not be as serious as it may seem, these media ought not to be chosen in the first place.

Petroleum products with a low content of aromatic compounds have up to now been most suitable; besides, they have (as a rule) no bad smell.

Some petroleum products of this kind are sensitive to oxidation by air. By oxidation more high boiling products and precipitates can be formed. Any trouble with oxidation by air has, however, not been encountered by present drying process. Probably this is explainable by the fact that the drying is carried out under vacuum, oxidizing air does not on the whole come in touch with the drying medium.

A petroleum product boiling between 187 and 212 C. has proved to be very suitable. It was uncolored, free from aromatic compounds and remarkably enough it had no odor. The flash point was 55 C. With this product it hasproved possible to dry wood fast and with maintained fresh color and with excellent recovery of the medium. The trade name of the product is Shellsol T.. Another similar product boils at between 19 1 and 257 C. The end boiling point of this distillate seems a little too high, and the boiling interval a little too wide.

The flash point was 66C. lts trade name is Shellsol K3. Other tested drying media which, however, have not been equally suitable to use have included dichlorobenzene; dichloropentane; tetralin; and many more.

To the selected drying medium other substances can be added for giving the wood different properties. Such are, for instance wax (which gives a water-repellent surface), bleaching substances (which give a lighter color to the wood), substances which give a hard surface such as resins and linseed oil; substances for protection against decay, blue stain or fire; also, color may be added. These additives or their effects more or less remain in the wood surface after the drying treatment.

In order to reduce foaming of the medium during the drying period one can but it seems not necessary add antifoaming agents to the medium; a good antifoaming agent is octylalcohol.

The drying temperature has in the example above been stated to be C. This temperature seems in most cases to be the most suitable. Of course the drying can be efiected at other temperatures. Thus, in the case of an easily damaged wood a drying temperature of 50 or 60 C. or even lower temperature can be used. The drying will then be much more time consuming. It is still, however, very fast in contrast to the very limited possibilities that in these cases prevail for drying in the conventional way. It seems un-necessary to use temperatures over C.

The vacuum (reduced pressure) ought to be sufficiently high that the water boils energetically, but not so high that the drying medium evaporates too fast. The drying proceeds best if the vacuum is as high as it can be with respect to the particular drying medium in use. The higher the boiling point of the medium is, the higher can and ought the vacuum to be; The pressure difference between applied vacuum and the vacuum corresponding to the boiling point of the water is a very important factor. This pressure difference has more resistances to overcome. It has not only to assist in the evaporation of the water and the removal of the steam:

It must also act a short distance into the wood. Within the wood the vacuum will obviously not be as high as outside the wood in the cylinder, the wood being of course not completely pervious. Even the vacuum within the wood ought to be sufficient to keep the water therein boiling. The vacuum must also be sufficient to draw the steam out of the wood. The importance of a high vacuum is further made clear by the fact that the temperature successively grows lower in the wood. A consequence of this is that the bigger are the wood dimensions the higher vacuum and temperature are wanted.

Another detail perhaps ought also to be mentioned. To reach the wood that lies some distance below the surface of the body of drying medium specially the wood that lies at the bottom of the cylinder the vacuum must overcome the pressure from the drying medium. This pressure is, however, rather low because the drying medium is mixed with a lot of steam bubbles: the specific gravity of this liquid-steam-mixture is comparatively low. Even if the drying conditions principally are not so good at the bottom of the cylinder as they are higher up, the differences are so small that as a rule they do not have practical importance.

The limits for the most suitable boiling points of the drying medium, the temperature of the drying and the vacuum thus depend on said circumstances. Other special conditions may also influence. Thus, for a cheaper drying medium it may be possible to accept some trouble with overdistillation if the medium should be easily evaporable; or, to accept some loss of medium remaining in the wood if the medium is difficulty evaporatable. Such things, however, are the exceptions.

As a rule it is easy to give limits for the most rational procedure when using a certain drying medium. In order to elucidate this and some connecting questions and facts above mentioned, the following table can give guidance. Steam pressure at 80 C. and at 60 C. for water and organic liquids boiling between 120 and Boiling point 80C. 60C.

IC. (water) 355 mm Hg 149 mm Hg [20 200 90 I60 50 20 I80 20 200 I0 3 220 5 2 Tern perature Pressure 25C. I 4 mm Hg 50 4.2 75 l0.6 I00 46.0

From the above it will be clear that substances boiling around 120 C. have comparatively high steam pressure at mentioned drying temperatures. The steam pressure is essentially lower for substances boiling at 160 C. A further considerable reduction in the steam pressure can be observed for substances boiling at 180 C., which substances can be used for drying at 60 C.,

perhaps also at C. In the last case, it is, however, desirable with higher boiling media. For substances with boiling points of about 200 C. the pressure decreases to a rational value for drying at 80 C.

At 80 C. the pressure of the water-steam is 355 mm. To have water boiling at said temperature the pressure has to be below the pressure value stated, a value which at 60 C. sinks to 149 mm. Thus, there definitely is a better possibility at 80 C. than at 60 C. to obtain the needed pressure differences in the vacuum. The steam needs these differences in order to overcome the inner resistance of the wood, the pressure of the medium, and the loss of pressure on the way to the cooler, etc. This means that the drying conditions are definitely much better at 80 C. than at 60 C.

It was mentioned, above, that the vacuum has to be as high as it can be with respect to the selected drying medium. How high a vacuum the drying medium resists is partly a question about how much medium that is allowed to distil over. It seems to be preferable that the pressure (the vacuum) is not kept lower than 2 to 4 times that of the steam pressure of the medium at drying temperature. Usually one employs an 80 to 98 per cent vacuum, i.e. under a pressure between 15 and 150 mm; a higher vacuum is selected for higher boiling media, and a lower for lower boiling media. At 60 C. it is more desirable with a high vacuum since the margins are less than at 80 C. When working with Shellsol T at a drying temperature of 80 C. a pressure between 20 and mm Hg has been used.

in order to be able to keep a high vacuum and to obtain a good drying it is necessary to arrange an effective cooling. By less effective cooling losses of heating medium via the vacuum pump arise and the pump will be heavily charged to carry away steam. Were the cooling effective no losses would arise and the charge on the vacuum pump would be very little (principally none at all).

A detail in this connectionis that the vacuum in the beginning of the drying period can be lower: then, it is successively increased to wanted value. In the beginning large quantities of water from the outer layers of the wood are evaporated. A heavy foaming may occur.

' This foaming can be reduced by an initial lower vacuum. An intense foaming usually increases the evaporation of the drying medium very much.

On the other hand, there seems to be no reason for having a low temperature in the beginning. Here there is a difference in contrast with conventional drying. If the temperature is kept high from the beginning most possible heat is delivered to the wood, which heat then is utilized when the vacuum is increased. Another question is that available heat capacity as a rule is not sufficient to reach highest temperature from the beginning. The starting temperature will then be lower for this reason. In such a case the lower temperature will contribute to a reduced foaming at the start of the drying period.

After the drying the drying medium is carried away from the cylinder with maintained vacuum as above stated. if the vacuum is broken while the drying medium is left in the cylinder the outer air pressure will press in large quantities of the drying medium into the evacuated wood.

After the removal of the medium the wood is allowed to remain in the cylinder with maintained or if possible increased vacuum. During this period the drying medium left in the wood evaporates at the same time as the temperature of the wood decreases. In the way the drying has been carried out there is only a small quantity of the medium left in the wood, the same being present in the surface layers of the wood. During the drying period the wood has received a considerable amount of heat. This is sufficient for the purpose of evaporating the drying medium mainly or completely. It is here to be observed that the large heat amount in the wood is well insulated. The cylinder is heat insulated and furthermore there exists a terminal insulating vacuum. in addition the wood has low thermal conductivity (good insulation) whereby heat in one place in the wood is carried away only slowly. The large amount of heat in the wood can thus practically completely be utilized for evaporation of the drying medium left in the wood.

Attention may be called to some differences in contrast to the situation when using conventional air drying. ln air drying it is of importance to have good space between the different wood pieces, and to arrange a powerful air circulation directed to all places in the cylinder. Contrary to this situation, the present drying has a certain similarity to the conditions obtaining in steam heating. The steam goes by itself to cold places and leaves there its heat contents by condensation, just where heat is needed. Furthenby' the condensation a vacuum is formed, new steam rushes in as long as heat is needed even if the passages are narrow. As soon as the drying medium evaporates by the local heat amount this means that an over-pressure in relation to environment is formed. The steam thus leaves the wood even if the passages are narrow.

Further advantage of the present drying process is that the medium has low vaporization heat: as a rule, it is only one fifth of the same heat of the water. Be-

I tween the wood pieces. During the drying the wood is surrounded by the medium. The medium has a high heat capacity. It enters into an intense stirring by the steam bubbles leaving the wood, which bubbles owing to the high vacuum in addition have a very large volume. Furthermore the wood and the drying medium have about the same weight per unit of volume, which cause there is a decreasing evaporation from the wood during this period it is easy to increase the vacuum successively with increased possibilities for the medium to leave the wood. Even if the treatment thus is very effective, this period may not be limited to short. It is an advantage, among other things, with respect to rain, moisture etc. that the food be allowed to cool down before it is taken out of the drying cylinder.

In order to facilitate very good evaporation condi tions for the medium left in the wood, different measures can be taken, if wanted. Besides maintenance or increase of the vacuum the temperature may also be increased at the end of the drying period. Further, hot air can be introduced in limited amount. This air can temporarily decrase the vacuum a little. Connected risks are small in this situation. Existing amounts of dry ing medium are now so small that they cannot be intro- -duced into the wood by an outer increased pressure.

They are absorbed by the cell wall and are not mechanically movable.

I Instead of air at this stage, steam can be introduced. A suitable measure in such a case is to introduce a quantity of hot water in the bottom of the cylinder, the heat supply being maintained. The water evaporates successively and exposes the wood to steam treatment. Such a steam treatment means a water-steam distillation of the medium. Treatment of this kind easily evaporates substances with comparatively high boiling points. The small quantitiesof the medium that eventually remain in the wood are comparatively high boiling.

explains why no, or only a low, pressure exists between the different wood pieces, which instead receive a certain movement in the medium.

At the beginning of the drying period the moist wood is as a rule heavier than the mixture of drying medium and steam bubbles. During the course of the period the wood dries and becomes lighter but the amount of steam bubbles in the drying medium decreases,- with the consequence that the mixture of medium-bubbles becomes heavier and heavier. The rather equal weights by unit of volume for wood and mixture of mediumbubbles thus meetleach other and are identical during a considerable part of the drying period.

An eventual arrangement with crossers (i.e., cross pieces) has as a rule more interest for the final evaporation of drying medium than for the actual drying of the wood. But, as said above, for the final evaporation of the drying medium a number of favorably acting factors exist. Furthermore, different actions could be made in order to obtain better and better results. No obstacle is at hand, however, to the use ofcrossers. If so they can be made thin and be used sparesely; they do not demand much space.

It is rather difficult to give exact'figures about the amount of medium overdistilled during the drying period. The amount is, to a high degree, dependent of the boiling point of the medium, the vacuum used and the drying time. By rational working only a little medium is distilled as the water evaporates. If, however, a medium with a low boiling. point is used, or if too high a vacuum is applied, the amount of evaporated medium increases very fast. The amount can be tenfold. Even if the medium is relatively easy to recover by the separation the amount of overdistilled medium ought to be kept low for reasons given above.

The amount of drying medium remaining in the wood also varies depending on the boiling point of the medium and measures taken. Remaining amount is of course more connected with the surface of the wood than with the volume of the wood. when the amount a of heat mainly is connected with the volume of the wood it is thus easier to remove the remainder of the drying medium from wood of heavy dimensions. Re maining amount of drying medium is as a rule less than 5 kilograms per cubic meter if drying media boiling at about 200C and a drying temperature around C are used. The amounts have been difficult to estimate with higher accuracy. It seems that the losses of drying medium need not exceed 2 kilograms per cubic meter of wood. The cost for this is very small.

The drying of wood 25 millimeters in thickness can when using rational process drying medium boiling around 200C. to 220C. and a drying temperature of 80C be fulfilled in 4 to 6 hours. If the thickness of the wood is doubled 50 mm the drying time is incrased to 6 to 8 hours. This means 25 to 50 times faster than drying in conventional apparatus with air as drying medium. The duration for the evaporation of the drying medium may be stated as being one or a couple of hours but (as said above) it is an advantage if it can be prolonged. The actual drying advantageously is carried out in daytime and the wood then is allowed to stay in the cylinder during night until next day in order to give off vaporized drying medium and to become cool.

According to the invention a drying medium is used in the form of a liquid at the drying temperature. This means real advantages. When the wood is surrounded by a liquid medium during the drying the risks for different surface damages will be very small. Perhaps it is so that the medium in part replaces the water in the surface of the wood during the drying period and in that way avoids the f6rmation of hardened spots and different defects. The drying medium jmay even have a certain comparatively seen delaying" effect on the water-steam evaporation from the very surface so that the wood in spite of a fast water-steam evaporation will be more evenly dried. In other words the difference in degree of moisture content between the inner and outer parts of the wood will not be so great as when drying by air. Another advantage is that the drying medium has sufficient heat capacity. The weight per unit of volume of a liquid medium is around 1,000 times higher than that of a gas at the temperatures here in question. The heat capacity is to a high heat-extent connected with the mass of the medium. With this high ing capacity it is possible to work easily even in vacuum, which is impossible when working with air.

The fastest way to dry wood undoubtedly is to raise the temperature so that the water boils away. On the other side may in spite of the protecting effect of the medium lowest possible temperature be used in order to avoid uneven drying with damages as a consequence. Both these view points are taken into consideration by carrying out the drying with a liquid medium under vacuum. Such a drying can according to the invention be effected with good effect and without disadvantages by taking special steps. In this connection it is observed that the drying medium may not penetrate the wood but only touches the surface. After finishing the drying the medium is 9emoved from the wood surface.

I claim:

1. Process for drying products of wood and other fibrous products in the piece, which comprises heating green wood at a temperature between 50 and C. in a closed container under reduced pressure in a water-immiscible drying medium having a boiling point of between and 250 C., and after the drying has been effected, removing the used medium from the closed container under reduced pressure.

2. Process according to claim 1, in which the drying medium boils at between 200 and 220 C.

3. Process according to claim 2, in which the drying medium boils at approximately 200 C.

4. Process according to claim 1, in which the drying is effected at a temperature betwen 60 and 80 C.

5. Process according to claim 4, in which the drying temperature is approximately 80 C.

6. Process according to claim 1, wherein the drying medium consists essentially of a petroleum product.

7. Process according to claim 1, in which after the drying period the product remains under reduced pressure in the closed container thereby evaporating drying medium from the surface of the product.

8. Process according to claim 1, in which the pressure is further reduced after removing the drying medium from the container.

9. Process according to claim 1, in which the temperature is increased at the end of the drying period. 

1. Process for drying products of wood and other fibrous products in the piece, which comprises heating green wood at a temperature between 50* and 90* C. in a closed container under reduced pressure in a water-immiscible drying medium having a boiling point of between 180* and 250* C., and after the drying has been effected, removing the used medium from the closed container under reduced pressure.
 2. Process according to claim 1, in which the drying medium boils at between 200* and 220* C.
 3. Process according to claim 2, in which the drying medium boils at approximately 200* C.
 4. Process according to claim 1, in which the drying is effected at a temperature betwen 60* and 80* C.
 5. Process according to claim 4, in which the drying temperature is approximately 80* C.
 6. Process according to claim 1, wherein the drying medium consists essentially of a petroleum product.
 7. Process according to claim 1, in which after the drying period the product remains under reduced pressure in the closed container thereby evaporating drying medium from the surface of the product.
 8. Process according to claim 1, in which the pressure is further reduced after removing the drying medium from the container.
 9. Process according to claim 1, in which the temperature is increased at the end of the drying period. 